1. Field of the Invention
This invention relates to improvements in transconductors, or the like, and methods for constructing same, and more particularly to improvements in methods and circuits for controlling the output impedance of a transconductor.
2. Relevant Background
A transconductor is a voltage controlled differential transconductance stage or circuit. Transconductance circuits are used in many applications, among which, for example is in driver circuits that supply drive power to a dc motor used to spin a disk of a hard disk drive, or the like.
In the construction of transconductor circuits, efforts are generally made to make the output impedance of the transconductor as high as possible. Traditional methods use a copy of the transconductor placed in an oscillator with feedback control so that oscillation is maintained at a constant amplitude. In this condition, the transconductor output impedance is nearly infinite. There are problems with this approach, however. For example, in such circuit, signals from the oscillator are inevitably leaked to the target transconductor. This is, of course, undesirable.
What is needed, therefore, is a differential transconductor which has a controlled high output impedance, and which does not leak oscillator signals to the target circuit.
In light of the above, therefore, it can be seen that one advantage of the invention is that a differential transconductor can be provided which has a controlled high output impedance and which does not leak oscillator signals to the target circuit.
This and other objects, features, and advantages will be appreciated by those skilled in the art from the following detailed description, taken in conjunction with the accompanying drawings and appended claims.
According to a broad aspect of the invention, a method is presented for controlling the output impedance of a transconductor having first and second differential inputs and a differential output. The method includes applying a dc voltage to the first differential input of the transconductor and to a first differential input of a differential difference amplifier. An inverted output of the differential output of said transconductor is applied to the second differential input of the transconductor and to a second differential input to the difference amplifier. The output of said transconductor is applied to a variable resistor that is controlled by an output of the difference amplifier.
According to another broad aspect of the invention, a transconductance circuit is presented which has a high output impedance. The transconductance circuit has a dual input differential transconductor having first and second differential inputs and a differential output. The first differential input is connected to receive a dc voltage. An inverting feedback path is connected between the second differential input and the differential output. A differential difference amplifier has a first input connected to receive the dc voltage and a second input connected to the inverting feedback path. A variable resistor is connected across the differential output. The output of the differential difference amplifier is connected to control the variable resistor to minimize the output of the differential difference amplifier.